1. Field of Invention
The invention relates to optical transmission filters for systems employing polarized coherent beams, and more particularly to filters exhibiting a differential phase delay to the components of a polarized beam element as a function of element position within the filter aperture. The invention has particular application to the far field correction of a laser beam.
2. Description of The Prior Art
The invention is applicable to optical systems, such as lasers employing polarized coherent radiation. In lasers, the optical resonator acting to provide optical feedback for the gain medium aids in establishing an internal beam and the two influence the characteristics of the beam produced. Since the external beam derived from the laser is derived from the internal beam, the resonator and the gain medium also affect the nature of the external beam.
An ideal characteristic of a laser apparatus is that it have a large natural aperture and produce a large high energy, high quality beam. The "larger" the beam, the higher the energy, and the smaller the far field beam divergence (ideally a minimum), the aperture being the critical parameter in defining this beam property. Beam quality is a relative term used to characterize a beam in reference to a standard beam. Beams resulting from operation of an optical resonator in a pure TEM.sub.00 mode for instance, may be represented as providing a standard beam referred to as "Gaussian". In a "Gaussian" beam, the intensity peaks in the center of the beam and gradually decreases to the margin of the beam. Meanwhile, the phase of the "Gaussian" beam remains relatively constant at the center of the beam and then changes rapidly at the perimeter of the beam to a large value leading to a phase reversal. Conversely, in an intensity profile of the beam, the phase is changing most rapidly where the beam intensity is approaching a minimum. In a beam showing evidence of multi-moding, the intensity may then reappear as a second fringe whose phase may be displaced 180.degree. from the phase of the central fringe.
In practical apparatus, the beams are often of substantially poorer quality than standard "Gaussian" beams, unless correction is provided. Typical issues in the design of an optical resonator, which influence beam quality, are whether the optical resonator is stable, unstable, or a combination of the two termed "stable/unstable". Typical issues in the design of the gain medium are whether the medium is circular or square or rectangular in cross-section, whether it has slanted end faces, (cut at the Brewster angle), and the presence of thermal focusing effects as the medium is operated. In all such designs, the quality of the beam is likely to suffer as the aperture of the system is increased or as the power is increased. In laser systems, polarized operation is frequently desirable in that it permits electro-optically "Q-switched" operation and aids in achieving improved laser operation, the improvement being in improved beam quality and increased power.
Accordingly, means for adjusting the phase of a wavefront may be of advantage in minimizing the size of a beam in the far field.